4 Common Failure of Piston Pump

Figure 1: MOOG piston pump of okmarts online

There are several common modes of failure for variable displacement piston pump. Understanding these modes of failure, the causes, warning signs, and the prophylactic means of monitoring will help you to reduce unscheduled downtime.

The four most common modes of failure in piston pumps from the following:

• Oil contamination
• Fatigue from transient pressure spikes
• Blocked or restricted pump inlet
• Pump case over-pressurization

These four modes account for the vast majority of all failures seen in the thousands of units inspected and repaired each year by the service technicians.

1. Oil Contamination

Contamination can be the result of many things, including egress of water, environmental contaminants, or component ware elements.

Telltale signs of contamination include:

• Valve plate scoring
• Vertical scratches on piston barrels
• Scratches on the face of pistons

Figure 2: Scratches on the face of pistons.

• Excessive wear of saddle bearings (or embedded particulate)
• Plugged orifices
• Stuck compensator spools
• Pistons being seized within the cylinder bore of the barrel

2. Fatigue from Transient Pressure Spikes

The fatigue that results from transient pressure spikes will present in the form of broken pistons (at the neck), shafts (where the cylinder barrel rides or at the tail), sheered control pins, damaged yokes, or the appearance of cracks between the kidneys of the cylinder barrel. Of the four common modes of failure, this is by far the most difficult problem to identify through electronic monitoring. It takes a device with an exceptionally quick scan rate as spikes can form and dissipate within the window of a few milliseconds.

Figure 3: Sudden increase in pump pressure.

If you suspect that your system is experiencing transient pressure spikes, hand-held monitoring devices are offered by many OEMs and are an excellent addition to any reliability team’s troubleshooting toolbox.

3. Blocked or Restricted Pump Inlets

Blocked or restricted pump inlets will typically present in the form of cavitation on the valve plate or what some service technicians refer to as a “marble sound." Vacuum at the inlet leads to the formation and collapse of small vapor-filled voids. The shockwave generated by the collapse of these voids leads to surface damage on the valve plate as well as the telltale sound.

Ways to monitor for this condition include the following:

• A limit switch mounted on a suction ball valve feeding the pump (a common system interlock)
• A pressure transducer installed into the suction line
• Vibration monitoring through the use of accelerometers mounted to the pump

Figure 4: Limit switch for piston pump.

4. Pump Case Over-Pressurization

Pump case over-pressurization can present in the half-moon gouging or full-moon scoring of the swashplate, damaged shoe retainer, bent seal retainer, or protruding shaft seals as well as shoe damage—rounded edges, loose fit on the ball, and excessive wear on the backs of the shoes. This kind of damage will be the result of shoe lift or rolling caused by the increased case pressure.

This condition can be monitored through the use of a pressure transducer in the case line. In practice, however, this is rarely done. The most common type of monitoring for case drains is flow observation. Monitoring for flow can give you a good indication of pump life—allowing you to see how efficient the pump is running (that is if 10% is leaving the case only 90% is going into the system). As a guide: Flow greater than 10% of total flow capacity of the pump indicates excessive wear. It is also worth noting that this method of monitoring pump life is not valid for pumps that have an internal check between the case and suction.

5. Final Thought

Understanding these modes of failure, the signs, root cause, and methods for identification will help to reduce unscheduled downtime and improve overall system reliability when it matters most.

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